585. Where does the white go when the snow melts? - JM
To start with, light slows down when it moves from air to ice and speeds up when it moves from ice to air. That's because the electric charges in matter can delay a light wave and slow it down. Since electric charges are more concentrated in ice than they are in air, light travels more slowly in ice than it does in air. Next, some light reflects whenever light changes speed. That's why a glass windowpane reflects some light from both its front and back surfaces. Similarly, light reflects from each surface of an ice crystal. Finally, snow is a jumbled heap of ice crystals. These clear crystals partially reflect light in all different directions like billions of tiny mirrors. The result is a white appearance. You see this exact same effect when you look at white sand, granulated salt, granulated sugar, clouds, fog, white paint, and so on. Each of these materials consists of tiny, clear objects that partially reflect light in all directions. Since they reflect all colors of light equally and spread that light in all direction equally, they appear white.
When the snow melts and becomes water, it stops having all those tiny surfaces to partially reflect light. Instead, it has only its top surface and this surface continues to partially reflect light. That's why you see reflections in the top of a puddle.